1. Field of the Invention
The invention relates to an image scanner, more particularly to an image scanner with an automatic signal compensation device which is used to compensate for instability in the brightness of the light output of a light source of the image scanner.
2. Description of the Related Art
There are two types of light sources conventionally installed in image scanners. The first type is a light emitting diode (LED) array which is incapable of emitting white light and is thus seldom used in color image scanners. The second type is a fluorescent lamp, such as a cold cathode fluorescent tube (CCFT) or a daylight lamp, which is ideal for use in color image scanners.
Referring to FIG. 1, whenever a fluorescent lamp is activated, the brightness of its light output gradually intensifies before stabilizing. The fluorescent lamp undergoes a brief transition period, usually four to five minutes long, during which the fluorescent lamp is gradually heated before the brightness of its light output stabilizes.
Referring to FIG. 2, the brightness of the light output of the fluorescent lamp varies in accordance with the period of use. The intensity of the brightness of the light output decreases after prolonged use of the fluorescent lamp.
Because of the instability in the light output of the fluorescent lamp, there is therefore a need to compensate for such instability so as to ensure proper operation of the image scanner.